The invention relates to a plate filter press which includes a frame provided with guide supports on which the filter plates, a pressure plate and a closing yoke bearing the closing device are mounted to be displaceable in longitudinal direction, the locking device also being in communication with an end plate which the filter plate stack contacts in its closed state.
German Utility Pat. No. 6,750,401 discloses a plate filter press of the above-mentioned type in which the end plate is formed either by part of the frame with the cylinder of the hydraulic closing device being fastened to a displaceably mounted locking yoke which is connected with the pressure plate via tie rods, or in another embodiment, the end plate as well as the pressure plate are displaceably mounted on the guide supports with the cylinder of the closing device being fastened to the end plate while the piston is supported on the press frame. This type of structure has the drawback that the pressure plate as well as the closing yoke or the end plate, respectively, are positively guided parallel to one another. Since, however, accurate parallel alignment of the filter plates in the closed state cannot be assured during operation due to soiling, particularly of the lower edge of the filter plates, but also due to manufacturing inaccuracies in the filter plates themselves, the filter plate stack, in the closed state, will not contact the end plate in a parallel manner regardless of whether the latter is rigidly connected with the frame, as in the first embodiment, or is displaceably guided at the guide supports. Due to the great closing forces involved, this inevitably results in a deformation of the press frame or at least of the plates connected together by the tie rods somewhat to the shape of a parallelogram. If this deformation is to be avoided, the frame and the plates connected by the tie rods must be made substantially rigid and will thus be very heavy. However when there is no deformation, the plates will no longer fully contact one another so that the individual filter plates are not sufficiently sealed against one another. Since a completely rigid frame cannot be produced with justifiable expenditures, this known plate filter press has the further drawback that the cylinder of the closing device is rigidly connected with the locking yoke or the end plate, respectively. Thus, during movement of the end plate or of the closing yoke, respectively, the sliding surface of cylinder and piston are stressed by canting forces so that not only the slide surfaces but also the seals are subject to great wear.
German Utility Pat. No. 6,803,483 discloses a further embodiment of such a plate filter press in which the connection of the pressure piston with the closing yoke as well as the points of connection of the tie rods with the displaceable pressure plate are effected via spherical bearings. With this arrangement the closing yoke as well as the pressure plate are able to change their vertical position or their rectangular position with respect to the suspending carrier so that they can adapt themselves to the position of the plate stack. Since, however, the plate stack is pressed by the pressure plate against a correspondingly rigid contact surface on the press frame, unavoidable transverse forces will develop when such a plate stack, which in itself is not parallel, is pressed against the end plate formed by the press stand, since all of the filter plates must be shifted against one another toward the pressure plate which is itself articulated at the tie rods. These shifting forces are proportional to the friction forces between the filter plates. These friction forces may become very high depending on the degree of soiling, the properties of the product and of the filter cloth and the material of the plates. Since the pressure cylinder is fastened to the end plate formed by the press stand on the side facing away from the plate stack and centered with respect to the plate stack, the reaction force of the pressure cylinder will also act on the center of the press stand. Since the plates are held in the press stand by guide elements which are each disposed above the center of the plates and since friction forces directed opposite to the reaction force consequently attack at this point, the plate stack will contact one side of the stand at the bottom. The filter plate stack thus automatically takes on the shape of a parallelogram during each closing process. Moreover, in the closed state this might cause a bending moment to act on the press stand which is transferred to the region where the stand portion, which constitutes the end plate, is fastened to the base. This, however, can be compensated only by increased expenditures during the construction of the press stand.